MOISTURE-CURING POLYURETHANE HOT MELT ADHESIVES: PREPOLYMER STRUCTURE VERSUS SYNTHESIS CONDITIONS

摘要

Moisture-curing polyurethane hot melt adhesives are gaining increasing market share in a variety of structural and product assembly applications. They offer the benefits of high green strength (versus curable liquid alternatives), excellent adhesion, and superior chemical and heat resistance in comparison to conventional non-reactive hot melts. These adhesives are based on isocyanate-terminated polyurethane oligomers (prepolymers) obtained by reacting di-isocyanates with diols at an equivalent ratio of about 0.5 - 0.67 alcohol/isocyanate. Thus high levels of free isocyanate groups remain. These groups react with moisture upon application to form the carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final cured adhesive product is thus a chain-extended urea/urethane, held together primarily via hydrogen bonding. These adhesives suffer from poor pot life even at relatively low application temperatures, typically 250℉. Viscosity increases at a rate of 10%/hr or more. The cause of the viscosity increase is believed to be side reactions with free isocyanate groups. The goal of this work is to study the side reactions that occur during polymerization and after extended heat aging, and methods to control them. This paper reports on an initial study using a model system. It is the first such study to quantify the molecular weight distribution and extent of side reactions in adhesive of this type as a function of reaction time and temperature.